On 103 A.D. emperor Trajan ordered to build a bridge over Danube river to be used for the crossing and supply of troops in the imminent Second Dacian War against Decebalus, for which he was preparing the biggest army since Augustus’ times, about 150,000 men.
The architect Apolodorus of Damascus, to whom the Pantheon is also attributed, was in charge of designing and building it. He was the emperor’s favorite architect because he also designed the baths, the forum, the market and Trajan’s column..
It was located near the current Romanian town of Drobeta-Turnu Severin, east of the Iron Gates, the natural canyon of the Danube that runs parallel to the border with Serbia. Its construction was completed in 105 A.D., constituting one of the most outstanding engineering works of antiquity.
It was 1,135 meters long by 14.55 meters wide and 18.60 meters high from the water, in an area where today the river is 800 meters wide. It extended over 20 pillars of brick masonry, mortar and pozzolanic cement joined by segmental arches of oak wood at intervals of 50 meters.
The bricks that form the pillars, some of which have been found, have great historical value, since the soldiers who participated in their construction carved the names of their units on them. For this reason it is known that the legions IV Flavia Felix, VII Claudia, V Macedonica and XIII Gemina participated in the construction of the bridge, as well as cohorts from the I Cretum, II Hispanorum, III Britorum and I Antiochensium.
On both sides of the bridge, castra (fortified camps, in singular castrum) were built, so that to cross the bridge it was necessary to go through the castra, remains of which are still visible today.
We know how the bridge was by the classical sources, and especially by Cassius Dio who, in his Roman History details its technical characteristics. But also because its representation appears in the Trajan’ s column at Rome. Even today, on both the Serbian and Romanian sides, the remains of the vaulted porticoes that gave access to the bridge can still be seen.
Trajan constructed over the Ister a stone bridge for which I cannot sufficiently admire him. Brilliant, indeed, as are his other achievements, yet this surpasses them. For it has twenty piers of squared stone one hundred and fifty feet in height above the foundations and sixty in width, and these, standing at a distance of one hundred and seventy feet from one another, are connected by arches. How, then, could one fail to be astonished at the expenditure made upon them, or at the way in which each of them was placed in a river so deep, in water so full of eddies, and on a bottom so muddy? For it was impossible, of course, to divert the stream anywhere.Cassius Dio, Roman History 68.13
Although the bridge only remained functional for a few decades, for more than 1,000 years it was the longest arch bridge in the world. It is not known exactly when, but there is evidence that Emperor Hadrian ordered the demolition of its upper structure to prevent barbarians from crossing it.
Some historians believe that it was only a temporary measure, and that the bridge could be used again during the reigns of Diocletian and Constantine. The Byzantine writer Procopius wrote that in his time, the 6th century AD, the bridge was already in ruins.
This, too, then, is one of the achievements that show the magnitude of Trajan’s designs, though the bridge is of no use to us; for merely the piers are standing, affording no means of crossing, as if they had been erected for the sole purpose of demonstrating that there is nothing which human ingenuity cannot accomplish. Trajan built the bridge because he feared that some time when the Ister was frozen over war might be made upon the Romans on the further side, and he wished to facilitate access to them by this means. Hadrian, on the contrary, was afraid that it might also make it easy for the barbarians, once they had overpowered the guard at the bridge, to cross into Moesia, and so he removed the superstructureCassius Dio, Roman History 68.13
The pillars were submerged, reappearing in 1858 due to the drought that brought the river’s flow down to levels never seen before. Two of them were demolished in 1906 to facilitate navigation. In 1932 there were 16 pillars left, but in 1982 only 12 could be found, possibly the others were washed away.